OBJECTIVE: To explore the viral etiology of mammary tumors in mice in order to have a model for human breast cancer research. APPROACH: The properties of the mouse mammary tumor virus (MuMTV) will be investigated. Each mouse strain carries a virus with slightly different biological properties. The structural components of several viruses will be compared. Immunological differences will be sought. The synthesis of viral components in cell cultures will be studied. RECENT PROGRESS: After removing the milk-transmitted MuMTV from high-mammary-tumor strains of mice, an endogenous virus is expressed in milk, usually after several litters. This virus is most commonly infectious in mouse strains different from those susceptible to the virus carried in high-tumor strains before foster-nursing.